In a rotary piezoelectric motor, also called rotary traveling-wave motor, the driving of a rotor is due to the friction of the teeth of the stator on the contact surface of the rotor, the motor comprising piezoelectric stator excitation means suitable for exciting the stator and causing the rotary movement of the rotor.
FIG. 1 of the appended drawings shows very schematically a simplified structure of a rotary piezoelectric motor limited to the principal members relevant to the invention. The rotor 1 takes the general form of a wheel having an annular contact pad 2 joined by a web 3 to a central hub 4. The stator 5 takes the general form of a stationary annular structure comprising an annular stator ring 6 which possesses a cogged surface 9 on which the contact pad 2 bears, which surface is supported while cantilevered to the outside, on an annular base 7 via a stator rim 8 extending substantially radially, the piezoelectric ceramic material 10 being fixed beneath the stator ring 6 on the opposite side from the cogged surface 9.
Thus, the rotor is driven by friction of the teeth of the stator 5 on the contact surface of the rotor 1.
FIG. 2 shows a pattern of the distribution of the piezoelectric elements beneath the stator ring 6 according to the prior art, the piezoelectric material 10 then taking the form of a piezoelectric ring with two excitation sectors, usually called “excitation electrodes”, namely a sector A corresponding to a sinusoidal excitation mode and a sector B corresponding to a cosinusoidal excitation mode, that is to say the piezoelectric sector A is excited under an excitation voltage of the k sin ωt type and the piezoelectric sector B is excited under a voltage of the k cos ωt type, k being a constant, t being the time and ω the period. The two excitation voltages are therefore offset by π/2 from each other and have the same excitation frequency.
Each excitation sector comprises a plurality of alternately biased piezoceramic segments a1-a6, b1-b6.
Thus, considering two adjacent alternate piezoelectric segments and by exciting them with the same voltage, one is made to contract and the other to expand. This results in the deformation of the surface of the stator 5, forming a standing wave, each piezoelectric segment of the piezoelectric sectors A, B having the same peripheral length corresponding to a half-wavelength λ/2 of the standing wave generated by the excitation of the piezoelectric sector A, B.
Thus, the excitation sectors A, B each allow a standing wave to be generated with the same wavelength λ.
The two piezoelectric sectors A, B are separated by a sector S which is not excited by an excitation voltage, with a peripheral length corresponding to a quarter-wavelength λ/4, these sectors being excited with excitation voltages offset by π/2 from each other.
Thus, the two standing waves generated by the excitation sectors A, B on the piezoelectric ring 10 are offset from each other by a quarter-wavelength λ/4.
Superposition of the two standing waves results in the formation of a traveling wave with a wavelength λ moving along the piezoelectric ring; consequently this wave also causes the deformation of the stator ring 6 to which the piezoelectric ring 10 is fixed, with the formation of a traveling wave moving over the stator ring 6.
By forming a traveling wave, it is possible to create small elliptical movements in the cogged surface 9 of the stator ring 6, which, by friction, causes the rotor 1 to rotate in a movement direction opposite to the direction of movement of the traveling wave.
The operating principle of a piezoelectric motor using a stator and a rotor as described above is well known (see for example U.S. Pat. No. 6,674,217).
The transmission of the rotation movement of the traveling wave to the rotor is based on the friction of the cogged surface 9 on the rotor 1; this friction, which has an efficiency of 30 to 40%, causes the temperature to rise, the temperature rise acting in particular on the internal stresses of the stator resulting from bonding the piezoelectric material to the stator.
Thus, the piezoelectric material reacts differently as to whether the temperature is higher or lower.
Owing to its being heated, it is therefore relatively difficult for the rotation speed of the motor to be controlled accurately during continuous operation of the motor.
Thus, if the user desires to operate the piezoelectric motor at a fixed rotation speed, the means for regulating the speed do not automatically take into account the temperature rise of the piezoelectric material, and the actual rotation speed of the piezoelectric motor is different from the required rotation speed.
The common practice is therefore to control the motor by controlling the excitation voltages of the two excitation sectors.
Thus, to modify the rotation speed of the motor over the course of time, it is common practice to modify the variable parameters of the excitation voltages, especially by modifying, independently or in a combined manner, the frequency and the amplitude of the excitation voltages and/or the phase and the amplitude of the excitation voltages.
However, this way of controlling the rotation speed of the motor as a function of at least two parameters is relatively complex to implement and does not take into account the modifications in temperature of the mode of operation of the piezoelectric ceramic.
It will therefore be particularly advantageous to be able to regulate the rotation speed of the motor according to a method that takes into account the temperature modification of the piezoelectric material.